Short Note

Identification of the Proton Drip Line

G.D. Alkhazov, K.A. Mezilev, Yu.N. Novikov, and A.M.Nurmukhamedov¹
Leningrad Nuclear Physics Institute of the USSR Academy of Sciences, Gatchina, Leningrad district 188350, USSR

N. Ganbaatar², K.Ya. Gromov, V.G. Kalinnikov, A. Potempa³, and F. Tarkanyi⁴
Joint Institute for Nuclear Research, Dubna, USSR

¹ On leave from the Tashkent State University, USSR
² On leave from the Mongolian State University, Ulan-Bator, Mongolia
³ On leave from the Nuclear Physics Institute, Cracow, Poland
⁴ On leave from the Nuclear Research Institute, Debrecen, Hungary

Received March 10, 1983

Abstract

A group of proton emitters of Au, Ir, Re, Ta has been identified by means of mass values derived from the experimental data. The proton drip line has been determined. It is shown that on the boundary of the proton stability the values of proton pairing energies increase by about 50%, as compared with the isotone nuclei near the beta-stability line.

Identification of the proton drip line may be reduced to the revealing of the cases where binding energy of the last proton Bp becomes negative Bp(Z,N)=M(Z-l,N) + m_p - M(Z,N)<0, where M(Z,N) is the nuclear mass, and m_p is the proton mass. Absence of experimental values for the masses did not allow one to establish the location of the boundary for the proton radioactivity. Direct measurements of the nuclide masses near the boundary are a very difficult task. Since the neutron deficient nuclides with mass number A>=150 are mainly alpha-emitters, mass determination be-comes simpler. The solution of the task is divided into two stages proposed in [1], i.e., to connect alpha-decay chains and measure nuclear masses in the ends of the chains. The first stage has been fulfilled in refs.[1-3], the second one - in ref.[4].

With the use of intrinsic Ge-detector the positron spectra of nuclides lying in the ends of alpha-decay chains have been investigated. The obtained end point energies of the positron spectra (mass differences of isobars) allowed one to determine the masses of nuclides in the ends of chains, such as 148Dy, 149Dy, 149Ho, 150Ho, 151Ho, 152Ho. Then it allowed one to determine through the known alpha-particle energies the masses of nuclides connected in the chains by alpha-transitions.

Our aims make us concentrate especially on the masses of odd-proton nuclei. Absolute values of mass excess deduced in this way for the even-even and odd-Z nuclei are given in the table. To make ourselves sure in reliability of the obtained results and employed reference data, we carried out a systematics for Bp and energies of alpha-transitions. The comparison between mass values based on alpha-decay energies and the data [5] obtained from the systematics of mass difference of isobar chains gave a satisfactory agreement.

The Bp values for some odd-proton nuclides calculated from the data of the table are presented in Fig. 1 where one can clearly see that 175Au, 169Ir, 171Ir, 165Re, and, perhaps, 161Ta must be proton emitters. Prolongation of the systematics made in Fig. 1 by dotted lines allow one to conclude that 176Au, 177Au, 170Ir and 166Re are also proton emitters and 178Au, 172Ir, 167Re, 156Lu are probable candidates for that kind of decay. Figure 2 presents a location of the proton drip line for some odd-Z nuclides. Using the adopted mass values given in our table (and in ref.[4]) one can calculate the values of proton pairing energies (odd-even mass differences)[6]:

Fig. 2. A part of the diagram of neutron deficient nuclides. Symbols: black circles - nuclides which masses determined in this work (see also [4]); open circles - nuclides with the known masses; black squares - stable nuclei; D and black circle inside - proton emitters identified by adopted masses; D and open circle inside - proton emitters obtained from extrapolations of Bp; D - proton emitters known from [7,8]. The nuclei in alpha-decay chains are connected with straight lines.

Fig. 3. Values of the proton pairing energies D p for isotones with N=84-92. black circles - our values; open circles - obtained from reference [5].

The obtained values of A are shown in Fig. 3, grouped according to isotones. One can clearly see that D p tends to increase as the proton excess grows in a wide range of mass numbers. The values of D p for the proton emitters are about 50% as large as the D p value for nuclides near the beta-stability line. Preliminary rough estimations of the neutron pairing energies for very neutron-deficient nuclei of this region allow us to assume the same trend, though neutron deficiency takes place here.

A similar behavior of D p and D n values seems somewhat amazing and be caused by influence of the proton stability boundary and Coulomb barrier (effective for protons, only) on the pairing processes .

The authors are thankful to Prof. A.A.Vorobiev for the support of this work and valuable comments. We are grateful to Professors E.Ye.Berlovich, V.A.Karnaukhov, L.A.Sliv and V.G.Soloviev for fruitful discussions.

        REFERENCES

          7. S.Hofmann, W.Reisdorf et al. Z.Phys., A305, 111 (1982).